1. Field of the Invention
The present invention relates to a level shifter, and more particularly, to a level shifter adaptive for use in a power-saving operation mode.
2. Description of the Prior Art
Historically, the primary mode of reducing power consumption in electronic circuits has been to insistently scale down the power supply voltage. Recently, a move to 1.8 V power supply has been popularized among low-power and high-speed circuit designers. However, problems may arise when a low-voltage integrated circuit is coupled to a high-voltage integrated circuit, or when the output signal of an integrated circuit having a low operating voltage swing is utilized for driving another integrated circuit having a high operating voltage swing. That is, when a front-end integrated circuit having a low operating voltage swing is coupled to a back-end integrated circuit having a high operating voltage swing, the operating voltage swing of the output signal generated by the front-end integrated circuit is required to be converted from the low operating voltage swing to the high operating voltage swing by making use of a voltage conversion interface.
Please refer to FIG. 1, which is a circuit diagram schematically showing a prior-art level shifter. As shown in FIG. 1, the level shifter 100 comprises a first transistor 112, a second transistor 114, a third transistor 116, a fourth transistor 118, a fifth transistor 120, and an inverter 190. The level shifter 100 receives an input signal Vin having a first operating voltage swing generated by a first circuit unit 181 and functions to convert the input signal Vin into a first output signal Vout and a second output signal Voutb both having a second operating voltage swing. The first output signal Vout and the second output signal Voutb have opposite voltage levels relative to each other.
In the circuit operation of the level shifter 100, a first supply voltage Vdd1 and a second supply voltage Vdd2 are required for performing related voltage level shifting operations. However, when initially powered, due to an occurrence of different powering delays, the second supply voltage Vdd2 is provided to the level shifter 100 either before or after the first supply voltage Vdd1 is provided to the level shifter 100. For instance, regarding a transient process during which the second supply voltage Vdd2 is provided and the first supply voltage Vdd1 is not yet provided, the third transistor 116 and the fourth transistor 118 are turned off and the first transistor 112 is turned on; meanwhile, the second supply voltage Vdd2 is forwarded to a node A via the first transistor 112 for turning on the fifth transistor 120. Then, the voltage at a node B is pulled down to ground voltage. Accordingly, under such initial powering situation, the second output signal Voutb is firstly set to be a high-level signal having voltage Vdd2, and the first output signal Vout is firstly set to be a low-level signal having ground voltage. That is, before the first supply voltage Vdd1 is provided, the level shifter 100 is capable of setting the first output signal Vout and the second output signal Voutb having opposite voltage levels relative to each other. If the fifth transistor 120 is omitted, the node B is floated before the first supply voltage Vdd1 is provided; in turn, the floated node B is likely to cause circuit malfunctions. According, the level shifter 100 is able to prevent an occurrence of circuit malfunctions when initially powered.
However, when the first output signal Vout is a high-level signal and the second output signal Voutb is a low-level signal during normal circuit operations of the level shifter 100 powered by both the first supply voltage Vdd1 and the second supply voltage Vdd2, if the first supply voltage Vdd1 is turned off for entering a power-saving operation mode, the node A is then floated in that the third transistor 116 is turned off. Similarly, the floated node A is likely to cause circuit malfunctions. That is, the level shifter 100 is not suitable for in use in a power-saving operation mode.